This is a really nice game, reminds me of mobile survivor.io. It is simple yet challenging and fun to play.
Among the suggestions I can say that due to some skill issues I was not able to progress much, but still I can mention few things:
1. You can add the score so the players can compete with themselves
2. Randomise the selection of powerups in the future
3. Consider making enemy health lower at the beginner stages and increasing it with each your upgrade, so it will be slightly easier to progress